Neon tubes, such as are used for signs, ornate displays and the like, require high voltages at relatively low current levels. Such voltages are on the order of several hundred to several thousand volts and stimulate the inert gas contained within the tube to the degree necessary to produce visible light.
Examples of power supplies used for such a purpose are shown and described in Sikora U.S. Pat. Nos. 4,682,081 and 4,800,323. The power supplies shown therein include a transformer with a primary winding, the current flow through which is controlled by a power transistor. The power supplies shown in the aforementioned Sikora patents use a voltage sensing circuit which is connected to the base of the power transistor. Such sensing circuit limits the current flowing in the primary transformer winding. The current limiting circuit is selected to prevent saturation of the magnetic core of the transformer. The Sikora power supplies are current mode controlled and employ capacitors connected in the secondary of the transformer. These capacitors are discharged during each power supply "cycle,38 thereby causing the connected stroboscopic light to be intermittently energized and emit a brief but brilliant flash of light.
Even though current mode controlled, the Sikora power supplies fail to recognize improvements in reliability which could be obtained. Such improvements result when the "turn off" signal to the main power transistor is maintained for a time slightly in excess of the time required to actually turn such transistor off as well as the propagation time delay of the current control network following detection of the peak of each current pulse.
Another power supply for gas discharge lamps is shown in Pacholok U.S. Pat. No. Re. 32,904, a reissue of U.S. Pat. No. 4,613,934. The circuit shown in the Pacholok patent uses a power transistor to control the flow of current through the primary winding of the transformer. The "turn-off" point of the power transistor is controlled by a switching transistor, the collector of which is connected to the feedback winding and the base of which is connected to the primary winding.
Yet another power supply for neon tubes is shown in Pacholok U.S. Pat. No. 4,698,741. The circuit shown in such patent uses two transistors in series to control primary winding current and a third switching transistor to turn off one of the foregoing two transistors, thereby preventing further flow of current in the primary winding.
Neither the Pacholok power supplies nor those shown in the Sikora patents recognize the advantages of a circuit which derives a control signal directly from primary winding current and uses such control signal to control the supply "turnoff" point based on a peak primary current, the amplitude of which may be selected by the user based on lamp brightness In addition, gas discharge lamps present a load, the capacitive time constants of which may change. The circuits shown in the foregoing patents fail to appreciate how a gas discharge lamp power supply may be arranged to substantially maintain the selected peak current in spite of changes in such time constants and consequent changes in oscillation frequency.
Further, the power supplies discussed above have failed to appreciate the advantages of using both terminals of a feedback winding for power supply control. More specifically, such prior power supplies do not appreciate the advantages of using one terminal of such winding to obtain an oscillator feedback signal and resolve it to an A.C. component used to supply a "turn on" signal and a D.C. component for control circuit bias and other functions. The advantages of using a second terminal of the feedback winding to provide sequential signals which alternately aid in turning on the power transistor and in turning it off are also unappreciated by the prior patents mentioned above.
Other shortcomings of earlier work in this field will become apparent from the description set forth below.